Method of and apparatus for powder elutriation



June 14, 1960 L. M. COWDEN ETAL 2,940,592

METHOD OF AND APPARATUS FOR POWDER ELUTRIATION Filed Feb. 23, 1955 Elufriafion Column Flow Control Panel Powder DIspenser .5 g 5: i I 1 .5 5 E 5 I L. o 2% a is .5 INVENTORS i i LEWIS M. COWDEN 2 L JAMES M. HENDARY 16m 3E BY X TTORNEY V United States PatentOl'ice METHOD OF AND APPARATUS FOR POWDER ELUTRIATION Lewis M. Cowden and James M. Hendry, Indianapolis,

Ind., assignors to Union Carbide Corporation, a corporation of New York Filed Feb. 23, 1955, Ser. No. 489,927

7 Claims. (Cl. 209-139) The present invention relates to a method of and ap paratus for powder elutriation and, more specifically, to the separation of sub-sieve powders according to particle size. I

It is the primary object of the present invention to provide a method of and apparatus for rapidly and efliciently processing large quantities of powders of sub-sieve particle size, whereby said powders are separated according to particle size. 7

It is another object of the invention to provide in a single pass operation a method of and apparatus for reducing the agglomeration of the powder particles prior to such separation to give amore efiicient separation of the size fraction desired.

Other aims and advantages of the invention will be apparent from the following description and appended claims.

In accordance with the present invention, the separation of powders in the sub-sieve size range into various fractions is accomplished by means or a continuous upwardly moving air stream. According to Stokes, particles having a terminal velocity (maximum falling velocity) greater than the velocity of an upwardly moving air stream will fall downward in the air stream. Particles having terminal velocities equal to the velocity of the upwardly moving air stream theoretically will remain suspended, while those having terminal velocities less than the velocity of the upwardly moving air will be carried upward. The terminal velocity of a solid particle in a fluid varies directly with the difference in densities of the particle and the fluid, and also directly as the square of the radius of the particle in accordance with the following equation derived by Stokes:

where V=Terminal velocity (cm/sec.) g Acceleration due to gravity (c'm./sec. )=980 d =Density of particles (gm./cc.) d =Density of medium (gm./cc.) (air=0.001) r=Radius of particle (cm.)

u=Viscosity of medium (poises) (air=0.000l83) In the drawing:

I Fig. l is a schematic view of apparatus embodying the invention; A

Fig. 2 is a partial vertical cross-sectional view of the elutriation column of Fig. l; and

Fig. 3 is a partial vertical cross-sectional view of a modified column structure.

Referring specifically to the embodiment of the drawing, elutriation column 10 is provided comprising closed outer duct 12 arranged substantially in the vertical position. An inner duct 14 is mounted in the substantially vertical position Within the outer duct 12 and forms an annular space 16 between the inner and outer ducts and a chamber 18 in outer duct 12 above the discharge end 2,940,592 PatentedJune 14, 1960 20 of inner duct .14. The lower end of inner duct 14 communicates through conduit 22 with heavy fraction collector 24 positioned below. An injector tube member 26 is substantially vertically positioned in inner duct 14, extends above duct 14, and discharges into chamber 18.

Conduit means 30 is provided for passing a stream of gas upwardly through annular space16 between outer duct 12 and'inner duct 14; Gas conduit 32 is provided for passing a stream of gas upwardly through the interior of inner duct 14, through the annular space 36 between injector tube 26 and internal walls of inner duct 14 to the discharge end 20 of inner duct 14 where it passes into chamber 18. Conduit 34 is provided for passing a gasborne stream of powder up through the injector tube 26 and discharging it into chamber 18.

A suitable gas, such as air, is supplied through conduit 40, containing regulator 42, to conduits 44, 46 and 48 containing valves 50, 52 and 54, respectively. One stream of gas passes through conduit 44, containing gas flow controller 56, to powder dispenser 58 where the powder to be separated is suspended in the gas stream which then passes through conduit 34 to the elutriation column 10. Concurrently therewith, the gas streams passing through conduits 46 and 48, containing flow controllers 60 and 62, respectively, pass to conduits 32 and 30, respectively, of the elutriation column 10. V F

Thevelocitiesof the gas streams within annular passage 16, passage 36 and chamber 18 are set to predetermined values, by setting gas flow rates and the dimensions of these passages so that the velocities at the desired points are either greater or less than the terminal velocities of the fractions of powder as determined by Stokes equation.

For example, should it be desired to separate fractions of a given powder composition lighter than 40 microns in diameter, the gas flow rates and dimensions of the passages must be so chosen that the gas velocity in annular passage 36 is greater than the terminal velocity of 40 microns-diameter particles of that composition. The velocity of the gas stream passing through outer annular space 16 will. in all cases be greater than that of the stream passing upwardly through inner annular passage 36 so that powder particles are not deposited in annular passage 16.

The discharge of the two concentricstreams of gas from annular spaces 16 and 36 into chamber 18 produces considerable turbulence in'the lower portion of chamber 18 up to a point at least beyond the discharge end of injector tube 26. This causes considerable and prolonged agitation of the powder in the lower portion of chamber 18 and serves to break up any agglomeration of particles that may have previously formed. In addition, a powder deflecting sphere 66 of hard rubber or 'the like is preferably provided at the conical discharge end 68 of injector tube 26. The impingement of the injected fluidized powder stream against this sphere at high velocity serves to further break up agglomerations of powder prior to discharge in the chamber 18. l

The upper end 70'of.chamber 18 is provided with outlet conduit 72 for carrying gas-home light fractions through the receiver 74 to an exhaust gas filter 76 through which the gas passes to the atmosphere leaving an accumulation of the light fractions of the powder on the internal surface of the filter. As this accumulation of light fractions builds up, it falls downwardly through receiver 74 to light fraction collector 78, positioned below. The

I exhaust gas filter 76 may be composed of any fine filter material, such as clo'th, through which the gas can pass and the light fractions of the powder cannot pass.

3 The height of the elutriation column 10 should be such that turbulence, due to the interaction of the three a gas streams, has abated well below the region in which home light was assessed to the light fraction receiver 74. I

In a modified embodiment, as shown in Fig; 3 of the drawing, inlet duct 14 is provided with outwardly tapered 7 upper discharge end 80. rather than the inwardly-tapered upper end 20 of Figs.' 1 and 2 of the'drawing. Such out- Wa'rdtaper is, preferable since this arrangement makes it more difiicultlforthe lighter fraction powders to enter annular space 36 or inner duct 14 and be carried down by heavier fractions intov the heavy fraction collector 24.

i Exchanging the: slope of the upper end of inner duct 14 in this manner, these lighter fractions-pass toward the Outer high velocity gas'strearn' in passage 16'. which carries them back up in chamber 18, thereby introducing a reworking action which increases the. eiiiciency of the elutnat n c m 7 r an example, apparatus. similar-11o that shown in velocity greater than the terminal velocity of said light fractions and less than the terminal velocity of said heavy h embodiment ofthe drawinglwasremployed to separate 7 subsieve powder of at tungsten carbide, igomposition. Three streams of air were employedm the elutriation column and powdertwas fed from the dispenser to the column at a rat .of' 2.0 lbs; per hour. Fractions of powder a lighter than 45 microns diameter were separated from the rest of the powder by employing an air flow in conduit powder-processed, theweightof'light frac; tion obtained in light fraction collector '18 wasr34.-2 ,lbs,'

' n "irid'erarie r of pow ers ha b pa e i QXQQEQQ wi h tho invention az high degree of enlar er; andhigh efiiciencies are. alsojobtainable in separating extremely smaH siZe fractions; flies, particles :10 8 lQ-ZO microns in diameter; f I Iris o f cgurse, tojbeunderstood that, when processing V ,7 of the lightffi'actionzsohtained in the light 7 tr etign colleetor-was found-to be.9,.6.9%; and, for SQlbs. 1

fractions; introducing'intosaid separating zone around said first stream a second upwardly moving gas stream having substantially uniformly across its entire cross-section a velocity greater thanthe terminal velocity of the heaviest fraction of said powder;injecting said powder to be separated into said separating 'zone'in the, region'of symmetrically positioned therem at the lower end'thereof powders oflower density, the velocities ofthe gaslstreams a pas ing throug the elutriation'column must be reduced to permit ,a il fi ientsuspension time for thepowders in herhamhe etth col mn in order to. attain; aneflicie'nt separat on conventional carrier gas may be em:

PiQYQd 3. 1 3 @Dlliitus of the invention as long as'it' is netre qti e w hthe powder to be Separated: It as n Q3114 fl li a m PIQ QEah c due to its easy availability and ascoiha dlin a a hatist ainied s: I

Y In the method ofseparating-subssieve powders into r xe nrd;lightand he w fractions according e ize hei pmvementwhicn comprises intro g ent ra sing essenarat ns' a t p y "gas strem hai iflglsuhstantially imiformly across its entire cressseetion a ve loscity .grea'ter thansthe terminal v y of said light iraet u s-t and less than the terminal ls c tyof said 'li avrirae ion i t uci i said pa a in zone a enndtsaid first ream a seeon'd u war mo n as. fitl'emsha fingzsubstantially uniformly a a oss its entire;cross;section-a velocity greater than'the i W19 mou h? hea e t fraction of said powder;

nd iaie ng sai Pa ns: te-he. separat d into saidisepa'rat: ing zone in the region of turhulenec caused by interaction ofsaid first'and secondIgaS streams, whereby said ishis ad hea y trast ens are snarted in that ub a tial y nnlvsaid light fira t n 'e y uprdly from saids nsleseparat ng o v t sasvistream peein ther from and substan ia y ysaid h r tra tionspass downwardly through said first gas stream while bein 'sus end s t e n Y 1 s- ;2. Method as claimed unclairn 1, in which the powder 7 to be separated isinjected Q69 .ally'and axially of the tw surro ndi g c n e r nqwd gran i s stre ms.

he method or "sep f 'a i les s ve Powders n pre e erm n ight an heavvr qti ns qe id ls to par icl es nmn sinss intro ms' nt asi sl s parat a a nr dete n a d gate u sa l fla id in lee to s d shanghai 19 t esi a of u l nce b the interaction of said' firstand second streams or gas; c ndu t me n a sum d with sa d interior ub for d aewde t eret r n h if. urbulenc pow 'Q saidp wq nd ondu @9261? i sd qf'said chamber for c nve n h ea e-bo n gh fract ons o said powder h re em to e a at and i h fraetiqn P wd collector.

App ratue n epar ting sub-sieve po er cserd:

ns o p le. n. q d nce w h c a nifl, herein r en ee' duit r s me -aid s r am i t g bc i w e hrou h aid '7 war y-b is mal et-i s n ra an ax al y wi h e p e 9 h @ondu t ean fo sur yin t two sfs eim n s ue i rie a nsv el streams. r j t 6. Apparatus for separating sub-sieve powders into a light fraction and'a heavyrfrac'tion according to particle size comprising in combination, an; outer substantially vertical gas duct; an inner substantially vertical gas duct symmetrically positioned therein at; thetlower end thereof and forming a first annular'space therebetween and a single separating ehamber=jthereahove; conduit means associated .with thelOWe'rQend of said outer .duct for ezro a s p a d y-@ 7g sass-imam h ving s b-- a ally u i m y, m ss 'iitsten re c oss ection a upwardly'through'sa'id first annular space between said outer and inner ducts to said chamber; the size of said first annular space being so selected that the predetermined a of said: fir gas s ream willr sult inf-T gas lo y through said first annular space being higher than the termin l Ysl city of the heaviest p ticl eing re an injector tube vertically positioned in said inner duct f rmingv a qqndannularjrspace therehetween and passing therefrom to discharge into said single separating chamber; conduit meansassociated with the'lower end of s aid inner duct for supplying a second stream of gas at a p ede ermined rate upward y h ough said ner vdu to sa d ehamber o form a r gi n of' turb l n y th the ipl fidete fmi led rate of said second gas stream will result in a gas velocity through said second annular space being higher than the terminal velocity of the desired light fraction particles and less than the terminal velocity of the heavy fraction particles; conduit means associated with said injector tube for passing a gas-borne stream of said powder thcrethrough to said single separating chamber; a powder collector positioned at the base of said inner duct for collecting the heavy fractions of said powder; and conduit means associated with the upper end of said chamber for conveying the gas-borne light fractions of said powder therefrom to a separator andlight fraction powder collector.

7. Apparatus according to claim 4, in which the charge end of said inner gas duct is outwardly tapered in shape to prevent fighter fraction powders from passing to said heavy fiaction collector.

6 References Cited in the file of this patent UNITED STATES PATENTS 253,344 Chichester Feb. 7, 1882 1,877,861 Hatch Sept. 20, 1932 10 2,754,966 Kollgaard July 17, 1956 Patent No 2340,592 June 14 1960 Lewis M. Cowden et a1.

It is hereby certified that error of the above numbered patent requiring Patent should read as corrected below.

appears in the-printed specification correction and that the said Letters Column 4,, line 2?, for "conduct" read duct Signed and sealed this 15th day of November 1960.

(SEAL) fittest:

KARL H AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents 

